Some Thoughts on Exposure in the Era of Digital Photography
Digital cameras are everywhere these days. Get out your cell phone to take a call and chances are you could take a picture as well. Point and shoot digital cameras that do everything for you but press the button are so common that the very idea of controlling exposure yourself seems foreign to many. While this sort of photography can be fun and is certainly convenient, your odds of getting the best results go up dramatically if you can take control of the exposure. But the while the rules of the game for exposure in this modern era of digital photography haven't actually changed, they have been amended to a degree.
A "full featured" camera should have some equivalent of matrix or evaluative metering, shutter priority and aperture priority, as well as fully manual metering modes. Regardless of how much control your camera lets you have, and how much you choose to exercise yourself, your camera is attempting to guide you towards a "medium" exposure that averages to eighteen percent gray. This may be fine if you are taking pictures of eighteen percent gray subjects, but can cause frustration if your subject is supposed to be significantly brighter or darker than medium. Who hasn't taken photos of snow only to have them turn out dull and gray?
There really is no such thing as a "correct" exposure. You can overexpose or underexpose sufficiently to make things come out as bright or as dark as you want. The question is then, what do you want? I don't know of anyone who truly wants gray snow, but if you do, your camera is more than willing to oblige. If you want the look of snow at night, you can underexpose and achieve that look even in broad daylight. Most often though, you probably want snow to look the way it does during the day: white. Your camera really doesn't want to do that since it thinks everything is medium gray, so it is generally necessary for you to take at least some control to get what you after.
Aperture and shutter speed are the two variables traditionally used to affect exposure. You can add a bit to either to increase exposure, or subtract some from either to decrease exposure. If you open up the aperture and want the exposure to stay the same, you have to shorten the time the shutter is open to compensate. This is basic exposure theory. But modern digital cameras provide a third variable in the form of ISO that is increasingly viable as a means of affecting exposure. It is not at all uncommon for current digital cameras to achieve great results at ISO speeds ranging over five or more stops.
Why is this important? While shutter speed affects exposure, it also determines how motion gets recorded in an image. Short shutter speeds freeze motion while long ones cause moving objects to blur. A picture of a waterfall looks entirely different with a change of shutter speed. And while aperture affects exposure, it also has a major effect on depth of field. Open the aperture up and the background blurs. Close it down and everything will appear in focus for vast distances. Objects in the distance will be clearly visible and may cause distractions at small apertures but may be completely lost in blur at wide ones. In contrast to these two primary variables for exposure, ISO speed has little impact on other image variables beyond exposure. Granted, if you raise ISO speed enough, digital noise can create serious problems, but within the range that the camera sensor can handle, you can think of ISO as affecting exposure alone. This means you can solve exposure problems that used to vex film photographers no end. Mountain wildflowers on a windy day used to be best revisited when the wind was calm, but now a modest increase of ISO can save the day, and the shot.
High ISO settings can change the experience of shooting at night completely. When all you had were ISO 100 or 200, night photography was only possible with wide apertures or cooperative, unmoving subjects. Shooting at ISO 3200 changes everything. Handholding with ordinary lenses becomes possible. It still strikes me as somewhat weird that I can do this sort of thing after considering it all but impossible not more than a couple of years ago.
Metering in the old days required the use of careful measurements before you pressed the shutter release, and careful patience afterwards as you waited to get your film back, hoping you hadn't ruined what you think will end up being a killer shot. Metering is still an important tool for getting exposure correct, but it's not the only tool. The histogram provides information not only about overall exposure, but also about the statistical distribution of brightness within an image. You can tell at a glance how much of your image is how bright. You want the histogram to match the scene. Not every shot should result in a bell curve. If you shoot something that has only dark and light tones and none in the middle, the resulting histogram should reflect this.
Sometimes I'm asked whether the histogram means that metering is now less important or even perhaps relatively obsolete. Not for me at least. Metering allows me to be prepared to take the shot. The histogram allows me to check that I got it right after the shot. One is proactive while the other reactive. It's worth noting though that some cameras do now allow for "live" histograms so you can use them before pressing the shutter release. Even still, I doubt they'll ever make metering by the photographer obsolete. While a histogram display includes data from the entire field of view equally, a meter can be set to spot metering or center weighted. This means you can accurately check how specific points in a scene will be rendered, not just what the exposure of image is overall.
Many digital cameras do provide a novel way to check for specific overexposed spots within an image in the form of "blinking highlights." Turn on a setting and you can view your image with an overlay that causes any burned out highlights to blink on an off so they stand out. If the only spots that blink are specular highlights on the edge of shiny objects or the glint of the sun through the trees where you don't expect any detail, all is well. But if the entire sky blinks on and off, you have a problem. The foreground may look perfectly fine, but a burned out white sky can spoil an otherwise winning image.
But perhaps the most revolutionary advance that digital provides is simply its ability to provide the photographer with instant feedback at all. Whether it is the histogram, the blinking highlight display, or simply the image review itself on the LCD screen, it can be a tremendous advantage to get feedback while you are still there in the field, in time to hopefully get another shot if the first one didn't work out as you thought it would. This also provides for the freedom to experiment without the fear and worry that you are wasting film or just wasting your time. If you want to see what a shot would look like somewhat over or underexposed for effect, go for it. Try things you don't even think will come out, just to see if perhaps some do. Try everything. You can delete what doesn't work before you get home and only you will know.